Question: Which of these numbers is prime? ${9,\ 21,\ 53,\ 57,\ 64}$
A prime number has exactly two factors: 1 and itself. The factors of 9 are 1, 3, and 9. The factors of 21 are 1, 3, 7, and 21. The factors of 53 are 1 and 53. The factors of 57 are 1, 3, 19, and 57. The factors of 64 are 1, 2, 4, 8, 16, 32, and 64. Thus, 53 is a prime number.